This invention relates to an air-fuel ratio control method and apparatus of an internal combustion engine. More particularly, the invention relates to an air-fuel ratio control method and apparatus for controlling the amount of fuel injected into the engine according to various signals which indicate the operating condition of the engine and according to an air-fuel ratio signal of the engine.
There is known a technique of performing feedback control for maintaining the air-fuel ratio (if an air-fuel passage from the intake passage through the exhaust passage located upstream of an air-fuel ratio sensor is defined as a working fluid passage, the air-fuel ratio is defined as a ratio of the amount of air actually fed into the working fluid passage to the amount of fuel actually fed into the working fluid passage) within a predetermined range by controlling the basic amount of fuel injected into the engine in accordance with various separate signals used for indicating the operating condition of the engine such as an air intake signal for indicating the quantity of air taken into the engine, a pressure intake signal for indicating the level of absolute pressure in an intake manifold of the engine and a rotational speed signal for indicating the number of rotations per minute or the rotational speed of the engine, and for correcting this basic amount of fuel to be injected into the engine in accordance with a detection signal from an air-fuel ratio sensor, for example, from an oxygen concentration sensor disposed in the exhaust system of the engine. According to this controlling method, it is possible to improve the exhaust gas purifying efficiency of a three-way catalytic converter disposed in the exhaust system of the engine. The reason for this is that the three-way catalytic converter which simultaneously reduces the three basic pollutants, CO, HC and NO.sub.x, exerts the highest degree of purifying efficiency when the air-fuel ratio is maintained within a narrow air-fuel ratio range in the vicinity of the stoichiometric air-fuel ratio.
In the conventional control apparatus of this type, however, since the detection response of the air-fuel ratio sensor is delayed when the engine is in a transitional condition and furthermore, since a time lag caused by the transmission of the air-fuel mixture from the intake system to the exhaust system exists in the engine, a problem sometimes occurs in that the air-fuel ratio feedback control cannot be carried out in response to the actual operating condition of the engine. Accordingly, in this case, the fuel injection amount is not corrected by the detection signal from the air-fuel ratio sensor, and thus, the fuel injection amount becomes equal to the basic injection amount calculated in accordance with various signals which indicate the operating condition of the engine. Therefore, while the feedback control is not carried, the air-fuel ratio of the engine coincides with the value determined in accordance with the basic injection amount. As a result, this air-fuel ratio deviates from the stoichiometric air-fuel ratio, the purifying efficiency of the three-way catalytic converter is reduced in proportion to this deviation, and large quantities of harmful pollutants in the exhaust gas are discharged from the engine.